1. Field of the Invention
This invention deals with a method of controlling the pigmentation of thermoplastic film. More specifically, the method involves measuring the apparent thickness of the pigmented film with a nucleonic thickness gauge and controlling pigment addition in response to a comparison between the thickness measured with the nucleonic gauge and the thickness determined from the weight of the film.
2. Description of the Prior Art
Thermoplastic film may be produced by feeding a thermoplastic resin, often in the form of solid pellets, to an extruder which mechanically works and heats the resin, transforming it into a plastic state. The molten resin is then extruded through an extrusion die to form a thin, thermoplastic film.
Pigment may be added to the extruder so that it is mixed with the thermoplastic material in order to vary the color of the extruded film. Such pigments generally represent less than 10%, and typically about 1%, of the total pigmented film weight.
As the pigment is quite expensive, normally costing several times as much as thermoplastic material, close control of the amount of pigment in each portion of the film is essential for economy of operation. Previously, the amount of pigment added was determined by visually observing the color of the product, thus allowing for a rather wide margin of error.
Another way of determining the amount of pigment in the pigmented plastic film is to weigh the film in order to ascertain if the film weight equals the total of the required amounts of film and pigment. However, this method is also inaccurate because it assumes that the thickness of the film is constant. This is not necessarily so and a very slight decrease or increase in film thickness, as might be caused by extrusion die wear or film buildup at the die exit, for example, will produce substantial variations in the amount of thermoplastic film extruded. A change in die setting of about 10 microns will alter the thickness of 500 micron film by 2.5%. As the entire amount of added pigment typically represents about 1% of the total weight of the pigmented film, a simple weighing of the pigmented film would not allow one to distinguish a decrease in pigment from a decrease in film thickness. Indeed, an excess of pigment and a deficiency of film could well cancel each other out, producing a total weight equalling that for the correct amounts of pigment and film but with an incorrectly pigmented film.
The thickness of the pigmented film could be measured mechanically, in conjunction with a measurement of the weight, in order to determine the variations in film thickness. A problem associated with the mechanical measurement of film thickness is that the thickness varies across the surface of the film sheet. This is obviously true of film produced from a rotating die which relocates the variations in film thickness produced by any irregularities in the die opening across the entire film as the die rotates. Thus, a large number of thickness readings would be required. Again, the errors produced by an excess of pigment and a deficiency of thermoplastic film could well cancel each other out resulting in a thickness which would appear to indicate a proper proportion.
The measurement of thickness by nucleonic or radiation gauges is known. U.S. Pat. No. 2,988,641 to Gough, for example, discloses a backscatter radiation gauge for measuring the thickness of calendered sheet material. The variations in the measured thickness can be monitored and used to control the setting of the calender rolls by a feedback loop. The thickness gauge makes a comparison with a set point controller which itself comprises a radiation gauge set to monitor changes in the composition of the sheet material. This patent does not, however, deal with the problem of regulating the amount of pigment in the material.
Continuous weight monitors for sheet materials are also known. For example, U.S. Pat. No. 2,726,922 to Merrill et al. discloses a continuous weighing scale which can be set to monitor variations from a predetermined thickness.